As well known, the grain oriented electromagnetic steel sheet which is mainly used as an iron sheet for the transformers is required to have a high magnetic flux density represented by B.sub.10 value as the magnetization characteristics, a low iron loss represented by W.sub.17/50 value, and excellent surface properties of a product steel sheet. In order to enhance the magnetic characteristics of the grain oriented silicon steel sheet as mentioned above, it is necessary to highly arrange the &lt;001&gt; axis of the secondary recrystallized grains of the product in a rolling direction. There have been heretofore huge improvements for this purpose, and it has now become possible to industrially produce a gain oriented silicon steel sheet having B.sub.10 value of larger than 1.89 T (Tesla) and an iron loss of W.sub.17/50 value of not higher than 1.05 W/kg. However, recently from the standpoint of the energy saving, it has been strongly demanded to lower the electric power loss than as before with respect to the electric appliances such as the transformers, and accordingly with respect to the grain oriented silicon steel sheet as the iron core material of the transformer and the like, there has been also demanded the one having a further lower iron loss value. In addition, there has been demanded to reduce the surface defects such as surface flawns and form an excellent insulating film with respect to the surface properties of the product.
On the other hand, the ultimate aim to have Si contained in the grain oriented silicon steel sheet is to increase the electric resistance of the raw material and to thereby lower the eddy current loss, that is, to reduce the iron loss. Therefore, to increase the content of Si is extremely effective to reduce the iron loss value. However, the increase in the content of Si leads to the problem that the surface properties of the steel sheet is deteriorated. That is, in the case of the process of making the grain oriented silicon steel sheet by using the AlN precipitation phase as an inhibitor, the slab is generally required to be heated at a higher temperature than in the ordinary steel prior to hot rolling in order to dissociate and solid-solve MnS to be coexistent as an inhibitor with AlN. However, if the slab is heated at such a higher temperature, it is likely that hot tear is produced during the slab soaking or hot rolling to develop the surface defects on the product. Particularly, if the content of Si exceeds 3.0%, the surface properties of the product conspicuously degrades with the rapid deterioration of the hot processability. Therefore, it has been so far compulsory that the content of Si is restricted to not higher than 3.0% so as to obtain the product having excellent surface properties. Thus, it has been considered to be actually difficult to reduce the iron loss value by further increasing the content of Si.